The use of renewable sources like vegetable oils in fuel production is steadily increasing. Since neat oils and fats of biological origin display inferior properties, there have been a considerable number of proposals for converting such materials into more satisfactory fuel products. A comprehensive overview of the prior art, which is hereby included by reference, is provided in EP 1 741 768 A1.
One possibility for improvement is the catalytic hydrotreating of animal and vegetable oils and fats to produce saturated hydrocarbons which e.g. may be useful as diesel and/or kerosene fuels. Such hydrotreating comprises the removal of unsaturations and the hydrodeoxygenation of the triglycerides. Due to the highly exothermic nature of these reactions, temperature control is very important to avoid undesirable side reactions. Such side reactions are further promoted by free fatty acids which are present in considerable amounts in animal and vegetable oils and fats. To mitigate these problems it is proposed in EP 1 741 768 A1 to subject such biological materials containing more than 5 weight % of free fatty acids to catalytic hydrotreating at a reaction temperature of 200 to 400° C. in the presence of a diluting agent, the ratio of dilution agent to fresh feed being 5-30:1. The dilution agent is preferably recycled products of the process.
However, the process proposed in EP 1 741 768 A1 still has some serious disadvantages. Thus, the amount of recycle to provide the necessary amount of diluting agent is very high. This constitutes a high hydraulic downstream load of the reactor and requires considerable revamp of existing units in order to provide the required enlarged reactor volume. Further, EP 1 741 768 A1 teaches to reduce the hydrogen consumption by promoting deoxygenation through decarb-reactions (formation of CO and CO2 from carboxylic oxygen) through suitable selection of the hydrotreatment catalyst. However, such deoxygenation of the triglycerides results in loss of valuable paraffin product, deactivation of catalyst due to the inhibiting effect of CO and high corrosiveness due to the presence of CO2.
Moreover, it is known that sulfide catalysts are active in respect of the following hydrotreatment reactions: hydrodesullfurization, hydrodenitrogenation, hydrodeoxygenation and hydrometallation.
Numerous writings deal with their potential in the deoxygenation reactions used for the catalytic conversion of bio-liquid (originating from oleaginous materials or lignocellulose) into fuel. In particular, Senol et al (Applied Catalysis A: General vol. 326, 2007, p. 236-244) have studied the conversion of a molecule of ester model type, representative of the hydrophilic function (ester group) and lipophilic function (alkyl chain) of triglycerides present in vegetable oils, in the presence of the sulfur catalysts CoMo or NiMo/Al2O3.